I. Field of the Invention
This invention relates generally to implantable cardiac rhythm management devices, and more particularly to a method for establishing optimum pacing site and intersite delay (i.e., AV delay and interventricular delay) parameters for a dual chamber implantable programmable pacemaker.
II. Discussion of the Prior Art
As is explained in the Baumann Patent 4,800,471 and co-pending application Ser. Nos. 09/545,536 and 09/734,282, assigned to the assignee of the present invention, and the teachings of which are hereby incorporated by reference, it is explained that cardiac pacing can be used to improve hemodynamics in congestive heart failure (CHF) patients. One indication of hemodynamic performance is reflected in the patient's pulse pressure (PP) which is defined as the difference between peak systolic aortic pressure and end diastolic aortic pressure. While PP could be used to optimize the pacing parameters in applying CHF therapy, a direct measure of PP would require the use of a suitably positioned pressure sensor inside the heart.
As is explained in the aforereferenced Baumann '471 patent and co-pending applications, an indirect indication of PP can be derived from the patient's atrial cycle length (ACL), which is the duration of the interval between consecutive P-waves in an ECG signal. The method disclosed in the above-cited patent/applications for using ACL to optimize CHF therapy parameters involves looking at a transient sequence in which, after a period of intrinsic cardiac activity, a short sequence of pacing stimuli is delivered to the patient's heart. Any subsequent transient increase in measured ACL provides an indication of the therapy's effectiveness over intrinsic cardiac activity. Likewise, a subsequent transient decrease in measured ACL is indicative that the pacing therapy is non-beneficial.
In applying the methodology to an implantable, microprocessor-based controller of the type typically used in a programmable dual-chamber pacemaker, the device is made to cycle through transient paced beats with different pacing site and AV delay configurations. Each such configuration is defined to be a group of consecutive beats with the same paced intersite delay and the same pacing site (right ventricular, left ventricular or biventricular pacing). As used herein, term “intersite delay” means the time interval between any sequential pacing events in the same cardiac cycle, regardless of whether they occur in different or in the same chamber. Each of the configurations is immediately preceded by a group of baseline beats. In the disclosed arrangement, three different pacing sites and five different intersite delays are used, with the AV delay of each such intersite delay being shorter than a previously measured value of the intrinsic AV delay. During bi-ventricular pacing, various interventricular delays are also tested. Interventricular delays provide variations in time intervals between pulse events with respect to pacing at multiple sites. It is common to stimulate both ventricle chambers, for example, and particularly the left ventricle can be provided with a plurality of sequentially paced beats. Each of these is operated using a timed delay sequence, which may be selected from a menu of sequence timings. The particular site and intersite delay configuration that results in the largest increase in ACL is then programmed into the pacemaker to thereby optimize hemodynamic performance of the patient's heart.
To avoid inaccuracies due to noise, the algorithm described in the Baumann '471 patent is made to vary the order of therapy randomization and averaging techniques are then used to extract data from repeated tests. While this approach has the effect of nulling out noise components, we have found that a significant portion of the unwanted noise in the ACL signal is due to respiration artifacts. To minimize the impact of respiration on hemodynamic parameters, such as ACL, in accordance with the present invention, the algorithm utilized in the Baumann '471 patent and the cited co-pending applications is modified. First, a respiration signal is derived, and then, on each iterative cycle when pacing pulses are applied following a period of intrinsic (baseline) cardiac activity, the delivery of the sequence of pacing pulses is synchronized to a predetermined phase of the derived respiration signal. Moreover, each test combination of pacing site and intersite delay uses pacing stimuli that are synchronized to the same phase of the respiration cycle. In doing so, noise due to respiration artifacts is essentially eliminated. As such, superior optimization of therapy parameters and improved hemodynamic performance are achieved. Further, by synchronizing the pacing pulses to the respiration waveform, a shortened testing and optimization protocol is made possible: Fewer repeated tests are required to obtain, after averaging, a specified noise level, since the noise due to respiration is reduced.